1. Field of the Invention
This invention relates to interactive display devices of the type which display data and allow an operator to input fresh data and/or change the data that is displayed. It is particularly concerned with such devices in which the data is displayed by an array of discrete display elements each of which can be independently driven to produce an optical output, and in which the interactive input is by means of a "light pen" or wand which detects light and which the operator points at the display element associated with the data to which the desired input relates.
2. Description of the Prior Art
Devices are known in which a matrix of display elements is scanned, the scanning circuit providing for each element a signal which determines the state thereof, i.e. whether or not it produces an observable light output and hence appears to be ON or OFF, and an identifying pulse signal whereby a circuit associated with the pen can identify the actual element at which the pen is pointed. The identifying signal must produce a light output from an OFF element, or absence thereof in the case of an ON element, which is detectable by the pen but not by the operator so that it does not appear to change the observable state of any element. This requirement imposes a limitation on the number of display elements which can be employed.
In said known devices the matrix of display elements is scanned step by step in synchronism with a shift register the state of each bit of which corresponds to the state of a corresponding display element, and sensing of light from a selected element by the pen necessarily operates to invert the value of the associated stored bit in the shift register. This is a functional limitation of the known devices.